i would start by playing a bunch of champs.. find with ones your like and have an affinity for.
i main
since day 1 in patch 3.6 she has always been my main, and always will be. she may not always be my highest winrate champ.. or the champ i play most in ranked due to meta shifts (or getting gutted). but even with a 45?% win rate.. in my elo (hey its not 42% anymore! YAY!!!). i can still carry game on her quite easily if my opponent don't get fed off mindless teammates
also consider the role u want.
are you really good at CSing? then top mid or ADC would be good...
if dont like CSing and would rather just focus on objectives and targeting the enemy champions with your skills then consider playing support.
but if u still want to BE the one to kill the enemies but dont like to CS try out jungle.
my main champs are
(main: jungle, primary role)
(main support, secondary role)
however i also play
and
as support
and i used to also play
and
in the jungle.
my previous mid laners also included
who i also really love playing. (but due to the fact i have been jungle/support i have gotten really rusty in my ability to CS.
other champs i play include
and
two very high skill cap champions. (although since last year syndra has been regarded a "slightly less" that "high skill)
there are remains other champs i rally LIKE but frankly cant play...
i absolutely love... but i really cant play her... it saddens me that i can't as i love her art her lore and her abilties and concepts... but.. im frankly just bad at her..
i also find really fun... but similar to ahri.. i cant play him really wish i could... but such is life...
saddest of all is
a champ i am EXTREMELY disappointed i cant play.. but as she is "half marksman" and ADC is the one role i cant play to save my life. no matter how hard i try and how much i would kill to be able to play her... i just cant do it.
i also liked due to her "kitty cat" skin, however i now despise seeing her not only becuase i cant play her... but because is is annoying a freaking play against as she is 100% impossible to land any skillshot on (at least if the kat know how to play properly) ever since shunpo was given a reset from her rework when picking up daggers.
so find a champions you like... and then find which ones u have an affinity for playing well.
overall i seem to prefer long range casters with skillshots, and/or gap closers.